Double text messages

I have an iPhone 6+ and my wife has an iPhone 5s. Recently her iMessages have been coming through as two separate text messages instead of all in one conversation. It's never done this before. Any tips on how to fix this? Thanks!

It could be a carrier issue of some sort. Try doing this:
1. Delete all message threads
2. Toggle off iMessage
3. Reset Network Settings
Settings>General>Reset>Tap Reset Network Settings. You may need to enter your wifi passcode back in afterwards.
4. Toggle iMessage back on